Instructions
Move your mouse cursor over the dart and click it once to pick it up. Your cursor will now turn into the dart, and moving your mouse will move the dart in your character's hand.
Place the dart cursor over the center of the dartboard, and then click and hold the mouse button down. You will see a red "X" appear over the spot of the dartboard where you are aiming.
Drag your mouse toward you while still holding down the button and your character will pull back on the dart preparing to throw it. Release the mouse button to throw the dart. If you pull back too far, the dart will go too high; if you don't pull back far enough, the dart will drop below the target. To hit the center of the dartboard, pull the mouse back so the bottom of the dart is just above the bottom of the screen before releasing it.